Question

  

Draw the structure of the diene that reacts with one equivalent of HBr to form the following two compounds as the only bromoalkene products.

Answer 1

Concepts and reason

In organic chemistry, alkenes are unsaturated hydrocarbons with at least one carbon-carbon double bond present in their structure.

Alkenes are more reactive due to presence of ${\rm{\pi }}$ -bonds. Alkenes undergo addition reactions (Example: hydro halogenation reaction, oxidations and polymerization etc).

Hydrogen halide on reaction with conjugated dienes undergoes an electrophilic addition reaction and leads to the formation of 1, 2- and 1, 4- addition product through the carbocation intermediate formation from alkene.

The type of products formed in the reaction can be monitored using the temperature.

Fundamentals

Electrophilic addition reaction: In this reaction, addition of reagent to alkene takes place, in such a way that the bond of olefin is transformed into two sigma bonds.

The general reaction for scheme for the addition of ${\rm{HX}}$ to butadiene is given below:

The reaction of dienes can affect the changes in the temperature.

oAt low temperatures, the reaction is irreversible and the kinetic product is formed. The substitution of halogen takes place through the formation of secondary carbocation (less substituted double bond in alkene).

oAt high temperatures, the reaction is reversible and the thermodynamic product is formed. The substitution of halogen takes place through more stable carbocation.(more substituted double bond in alkene).

The reaction is as follows:

At higher temperature, the 1, 4-addition product is formed. This product is also called as thermodynamic product (stable).

At lower temperature, the 1, 2-addition product is formed. This product is also called as kinetic product(less stable).

Ans:

The structure of the diene that reacts with one equivalent of ${\rm{HBr}}$ to form the following two compounds as the only bromoalkene products is given below: